Understanding antioxidant mechanisms in preventing oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 230

group and the subsequent radical is stabilized by resonance delocalization
throughout the benzene ring structure. The effectiveness of phenolic FRS can be
increased by substitution groups. Alkyl groups in the ortho- and para-positions
enhance the reactivity of the hydroxyl hydrogen towards lipid radicals, bulky
groups at the ortho-position increase the stability of phenoxy radicals and a
second hydroxy group at the ortho- or para-position stabilizes the phenoxy
radical through an intramolecular hydrogen bond (Shahidi, 1992). Below are
examples of some of the most common FRS in foods.
10.2.1 Naturally occurring phenolic free radical scavengers
Tocopherols
Tocopherols are a group of 8 different homologs that have a hydroxylated ring
system (chromanol ring) with a phytol chain. Differences in tocopherols are due
to different degrees of methylation on the chromanol ring with being
trimethylated, (positions 5 and 8) and (positions 7 and 8) being dimethylated
and being monomethylated (position 8). Tocotrienols have 3 double bonds in
the phytol chain while the phytol chain of tocopherols is saturated. Tocopherols
have 3 asymmetric carbons on the phytol chain and thus each homolog can have
8 possible steroisomers. Natural tocopherols are found in the all rac or RRR
configuration. Tocopherols originate in plants and eventually ending up in
animal foods via the diet (Parker, 1989).
The initial interaction of tocopherol with a free radical results in the
formation of tocopheroxyl radicals. Two tocopheroxyl radicals can react to form
tocopherylquinone and a regenerated tocopherol (Nawar, 1996). Formation of
tocopherylquinone is also thought to occur by the transfer of an electron from a
tocopheroxyl radical to a phospholipid peroxyl radical to form a phospholipid
peroxyl anion and a tocopherol cation. The tocopherol cation hydrolyzes to 8a-
hydroxytocopherone which rearranges to tocopherylquinone (Liebler, 1992).
Under condition of extensive oxidation, high concentrations of peroxyl radicals
can favor the formation of tocopherol±peroxyl complexes via radical±radical
termination reactions. These complexes can hydrolyze to tocopherylquinone. Of
less importance are interactions between tocopheroxyl and peroxyl radicals
which form an addition product ortho to the phenoxyl oxygen followed by
elimination of an alkoxyl radical, addition of oxygen and abstraction of a
hydrogen to form two isomers of epoxy-8a-hydroperoxytocopherones. Sub-
sequent hydrolysis leads to the formation of epoxyquinones (Liebler, 1990,
1991). Formation of epoxide derivatives of tocopherol represents no net
reduction of radicals (due to the formation of an alkoxyl radical) and a loss of
tocopherol from the system whereas formation of tocopherylquinone can be
regenerated back to tocopherol in the presence of reducing agents (e.g., ascorbic
acid and glutathione). An additional reaction which can occur is the interaction
of two tocopheroxyl radicals to form tocopherol dimers (Draper, 1967).
Generally, -tocopherol is the most reactive and less stable form of
tocopherol followed by -, -, and -tocopherols. This is related to their bond